Did you know that there is a very slight bug in the radiosity code for all versions after 1.80? Vertex colors are multiplied by the current RGB values of the material assigned to a mesh. In v1.8, blender created a new material with default RGB values and set the VCol Light flag. After 1.8, the same material colors are kept, and this results in very incorrect illumination. So here’s a simple fix:
for material in Blender.Material.Get():
Yep, run it after the radiosity calc. And it will change all materials so be careful.
The “bug” could make colors appear washed out, depending on what the original color was. Try on my cornell box example. The point is that meshes with radiosity should have a grey/white/neutral color, not a deep red, for example.